1. Field of the Invention
The invention relates to a method for monitoring a tank venting system for trapping fuel vapors and feeding the fuel vapors to an internal combustion engine having a fuel tank and an intake tube, wherein the tank venting system includes a container adsorbing fuel vapors, a venting line connected between the fuel tank and the container, a regeneration line connected between the container and the intake tube of the internal combustion engine, a ventilation line communicating with the atmosphere and the container, a ventilation valve for closing the ventilation line, a tank venting valve being disposed in the regeneration line and being triggered in an opening direction for feeding fuel vapors stored in the container, and a pressure sensor detecting a system pressure of the tank venting system.
The purpose of such a tank venting system which is known, for instance, from German Published, Non-Prosecuted Application DE 40 03 751 A1, corresponding to Published International Application WO 91/12426 and U.S. Pat. No. 5,193,512, is to prevent hydrocarbons from evaporating out of the fuel into the atmosphere.
To that end, the tank venting system has a fuel tank and a tank venting valve that communicates with the engine intake tube, so that with the aid of the negative pressure prevailing there, the fuel vapors can be extracted by suction out of a container located between the tank and the tank venting valve during certain operating states. To that end, a ventilation line, mounted on the bottom of the container filled with the activated charcoal filter, is provided with a controllable shutoff valve (ventilation valve). The activated charcoal filter located in the container adsorbs fuel during periods in which no extraction by suction from the intake tube takes place.
Due to the danger in such tank venting systems that some parts of it may leak, or that the tank venting valve may not function properly, the tank venting system must be repeatedly checked for functional capability during vehicle operation.
In order to check the tightness or leakproofness of the system, the ventilation valve is closed, and a pressure sensor, mounted in the fuel tank, for instance, measures whether or not the requisite negative pressure for checking has built up in the tank. If so, the conclusion is drawn that the system can function.
However, the pressure sensor may also be disposed at some other point of the tank venting system, such as in the line between the tank venting valve and the activated charcoal container, or in the line between the activated charcoal container and the fuel tank, or in the ventilation line upstream of the ventilation valve.
In order to prevent an overly high or an overly low pressure from building up in the tank venting system if the ventilation valve is not functioning properly, the system of German Published, Non-Prosecuted Application DE 40 03 751 A1, corresponding to Published International Application WO 91/12426 and U.S. Pat. No. 5,193,512 is provided with a mechanical protection valve assembly in the ventilation line. That mechanical protection valve assembly has an overpressure protection valve and a negative pressure protection valve, and the pressures of the valve assembly are adjusted in such a way that there is no danger of damage to the tank venting system from overly high or overly low pressures.
That apparatus has the disadvantage of requiring not only the ventilation valve and the tank venting valve but also additional mechanical valves in the lines.